Exhaust gas purification systems employing ceramic catalytic converters are well known as means for removing the carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx) contained in exhaust gas from automobile engines. A ceramic catalytic converter basically comprises a ceramic catalyst carrier (this is usually called the “catalytic element”) in the shape of a honeycomb, for example, housed in a metal casing, or housing.
As is known, there exist many different types of ceramic catalytic converters, but the usual construction employed has the gap between the housed catalyst carrier and the casing filled in with a heat insulating material typically composed of a combination of inorganic fibers with organic fibers and/or a generally liquid or paste-like organic binder as disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) Nos. 57-61686, 59-10345 and 61-239100. The heat insulating material filling in the gaps thus holds the catalyst carrier, and can prevent unexpected mechanical shock due to impact or vibrations from being exerted on the catalyst carrier. Since the catalyst carrier is not destroyed or shifted in a catalytic converter with this type of construction, the desired operation can be realized for extended periods.
Incidentally, it is preferred for catalytic converters to be operated at higher temperatures in order to improve the exhaust gas purification and enhance combustion. Particularly in recent years, with the movement toward tougher standards for exhaust gas with the aim of protecting the earth environment, there has been a trend toward more efficient purification of exhaust gas by increasing the operating temperature. In fact, operating temperatures for catalytic converters have reached as high as 800-1000° C., and even higher. However, insulating materials such as those disclosed in the aforementioned unexamined patent publications cannot be applied for operating temperatures in such high ranges, because of their composition and others.
Attention has recently been directed toward insulating materials composed mainly of crystalline alumina fibers, which can withstand high operating temperatures, and products using them have been implemented. As one example, Japanese Unexamined Patent Publication (Kokai) No. 7-286514 discloses a holding material (corresponding to an insulating material) for use in an exhaust gas purification apparatus characterized by being composed of a blanket wherein crystalline alumina fibers are arranged in layers, and sections of the fibers are oriented in the direction normal to the layer surface by needle punching. The alumina fibers used for this holding material must have a mullite composition with an alumina to silica weight ratio of 70/30 to 74/26. If the alumina to silica weight ratio is outside of this range, deterioration of the fibers occurs more rapidly due to crystallization and crystal growth at high temperature, such that the material cannot withstand use for extended periods.
Nevertheless, with the exhaust gas purification apparatus holding material described in Japanese Unexamined Patent Publication (Kokai) No. 7-286514, not only are the desired function and effect exhibited only when using alumina fibers having a mullite composition in the aforementioned limited range, but using alumina fibers with a low mullite ratio (for example, a few percent) results in increased plastic deformation and poor bearing retention (compression resistance) in high temperature ranges. Furthermore, since the compression resistance tends to rapidly decrease as a result of use at high temperature even if the compression resistance at the initial stage is satisfactory, it is very difficult to maintain high compression resistance for extended periods. On the other hand, when using alumina fibers with a high mullite ratio (for example, 75% or more), the compression resistance in high temperature ranges is improved because of reduced plastic deformation, but the higher brittleness which also occurs results in easier breakage of the alumina fibers, such that it is impossible to avoid worsening of “wind erosion” (a phenomenon in which the holding material crumbles at both ends due to wind pressure). As this wind erosion continues to progress, it causes dwindling of the surface area of the holding material which is supposed to exhibit holding force, and this leads to lower overall compression resistance and thus inconveniences such as shifting of the catalyst carrier. These properties are mutually opposing, and it is preferred to provide an exhaust gas purification apparatus holding material that simultaneously satisfies both properties of high compression resistance in high temperature ranges and its maintenance, together with excellent wind erosion resistance.